1. Field of the Invention
The present invention relates to an image forming apparatus for developing an electrostatic latent image formed on a photosensitive medium, such as a photosensitive drum, into a toner image in office machines, such as a laser beam printer, a facsimile, a digital copier, etc., employing an electrophotograph method, and more particularly, to an image forming apparatus having a developing gap detecting function capable of correctly detecting a developing gap between a photosensitive medium and a developer conveyer, such as a developing roller, at a low cost.
2. Description of the Related Art
FIG. 1 is a view schematically showing a general developing device. Referring to FIG. 1, the developing device includes a photosensitive medium 1 (hereinafter, called a photosensitive drum) forming an electrostatic latent image or electrostatic latent images by a laser scanning unit (LSU), not shown, using a voltage level property of a surface thereof, a charged roller 2 applying an electrical property to the surface of the photosensitive drum 1 while rotating to be in contact with the photosensitive drum 1, a developer conveyer 5 (hereinafter, called a developing roller) depositing a developer (toner) which includes desired colors of toner to the electrostatic latent image formed on the photosensitive drum 1 while rotating in a direction opposite to the photosensitive drum 1 to form a visible image, a developer supplying roller 6 supplying the developer to the developing roller 5, a developer amount regulating member 7 regulating an amount of a developer layer of the developer deposited on the developing roller 5, a cleaning blade 10 removing a residual developer remaining on the surface of the photosensitive drum 1 after one period of rotation of the photosensitive drum 1, and a power supply supplying a supply voltage to the photosensitive drum 1, the developing roller 5, and the developer amount regulating member 7.
An operation of the image forming apparatus having the above-mentioned developing device is described as follows. Firstly, the surface of the photosensitive drum 1 is charged uniformly to a desired voltage by the charged roller 2.
Thereafter, the LSU converts digital signals inputted from a computer or a scanner through a laser diode into a laser beam in a form of optical signals and emits the laser beam to the photosensitive drum 1, thereby forming the electrostatic latent image on the surface of the photosensitive drum 1.
The developer supplied on a surface of the developing roller 5 is moved in a developing gap g formed between the photosensitive drum 1 and the developing roller 5 through a rotation of the developing roller 5. At this time, the developer is maintained in a desired developer (toner) thickness on the surface of the developing roller 5 by the developer amount regulating member 7 mounted in an upper part of the developing roller 5.
Thereafter, during a rotation of the photosensitive drum 1, the developer is moved to the electrostatic latent image of the photosensitive drum 1 by a voltage level difference between the electrostatic latent image on the photosensitive drum 1 and the surface of the developing roller 5, and the electrostatic latent image formed on the surface of the photosensitive drum 1 is developed to a visual form of a toner image (visual image).
If a sheet of paper is fed between the photosensitive drum 1, on which the toner image is formed, and a transfer roller (not shown) located under the photosensitive drum 1. The transfer roller generates a high voltage of air discharge to transfer the toner image deposited on the photosensitive drum 1 onto the sheet.
Thereafter, the photosensitive drum 1 continues to rotate, and the cleaning blade 10 removes the developer remaining on the surface of the photosensitive drum 1 to enable the photosensitive drum 1 to form a next electrostatic latent image or next electrostatic latent images. At this time, the sheet to which the toner image is transferred, is fixed by heat and pressure and then discharged out of a machine. Accordingly, a series of image formation processes is ended (completed).
However, it is important for this image forming apparatus to uniformly keep the developing gap g between the photosensitive drum 1 and the developing roller 5 to maintain a developing quality uniformly and stably in an operation of depositing the developer on the electrostatic latent image of the photosensitive drum I to develop the toner image.
In order to achieve this goal, as shown in FIG. 2, the developing device 1 includes a spacer 5a having two spacer rolls as a device for maintaining the uniform developing gap g between the photosensitive drum 1 and the developing roller 5, wherein the two spacer rolls are capable of rotating to be in contact with the surface of the photosensitive drum 1 at both ends of a shaft 5b of the developing roller 5.
Since the spacer 5a has a greater external diameter to form a desirable developing gap g with the photosensitive drum 1 than an external diameter of the developing roller 5, when the photosensitive drum 1 and the developing roller 5 are disposed opposite to each other with respect to the developing gap g and rotated at a uniform linear velocity by a photosensitive drum gear 1a and a developing roller gear 5c, the developing gap 9 between the developing roller 5 and the photosensitive drum 1 is always maintained uniform. As shown in FIG. 3, the developing gap g is expressed by a formula (D2-D1)/2-(D4-D3)/2 wherein D1 is an external diameter of the developing roller 5, D2 is an external diameter of the spacer 5a , D3 is an external diameter of the shaft 5b, and D4 is an internal diameter of the spacer 5a. 
However, in respective developing devices, the developing gap varies depending on measurement precisions of related parts. Image qualities also vary in accordance with variations of the developing gaps. With a large developing gap, a developing electric field becomes weaker, and accordingly, an image density is lowered. On the other hand, with a smaller developing gap, the developing electric field becomes stronger, and accordingly, the image density becomes higher. In a worse case, there can be a discharge inducing an image noise. Accordingly, to solve such a problem, it is required to precisely install the developing device in the image forming apparatus, sense the developing gap of the developing device upon printing, adjust the developing electric field appropriately according to a sensed result, and outputting the variable electric field.
For this purpose, a technology for forming reference images on a photosensitive medium or a transfer belt and detecting an image concentration using an optical sensor has been disclosed. However, this conventional technology has a disadvantage that due to a high cost of the optical sensor, the production cost increases.
Additionally, U.S. Pat. No. 5,521,683 discloses an apparatus for detecting a developing gap by applying a constant voltage and a constant current to a developing conveyer. However, this reference has a disadvantage that a voltage and current variation corresponding to the variation of the developing gap is too small, and thus becomes inaccurate.